The present invention relates to a process for forming a grid of polymeric material. Such grids are usefully applied to sites in need of water and soil protection such as slopes, retaining walls, protecting walls, protecting banks, breakwaters and wharves.
More than 3,000 years ago pre-stretched (prestressed) earth and clay materials making use of a tensile member were known and materials such as reed and rattan were applied to many large scale earthworks to reinforce clay, brick and granular soil. In 1963 the French civil engineer Henri Vidal developed a modern process for producing pre-stretched (prestressed) earthwork using linear pre-stretched strips of high tensile strength by taking advantage of selected granular soil as determined by his dynamic analysis. The basic concept of producing pre-stretched earth and clay materials was derived from the realization that the friction force generated from the mutual action between the contact faces of both materials, i.e., the soil in pre-stretched earth and the pre-stretched material, can resist the relative movement of these two materials. Thus, the pre-stretched material can prevent the pre-stretched earth material from being laterally deformed and provide it with a so-called apparent anisotropic cohesion which is proportional to the soil density and the tensile strength of the pre-stretched material. Based on the foregoing principle, the pre-stretched material applied to the filled earth may be in the shape of a plate, grid, network, tube, rope, bar, rod, chain, etc. So far as the nature of the material is concerned, regardless of whether it is an alloy, copper, galvanized carbon steel, stainless steel, reinforced glass fiber, polymer, wood or other material, if it has suitable tensile strength, friction resistance against the earth or clay material will be achieved, providing considerable durability and economy.
A previously proposed pre-stretched grid of polymeric material is a network polymer with high strength for application to a pre-stretched earth or clay material. The polymer generally used was a strong engineering material made from processing specially selected polyolefins and polyesters and particularly suitable for the purpose of long-range pre-stretching without any adverse chemical reaction.
A conventional polymer pre-stretched grid production has to use considerably complex equipment and a prior process is as follows:
(1) feeding polyethylene (or polypropylene or polyethylene terephthalate or other suitable polymeric material) into a molding machine for molding to make a thin polymer sheet; PA1 (2) feeding the thin polymer sheet into a punching machine for evenly punching to form a thin polymer network sheet; PA1 (3) feeding the punched thin polymer network sheet into a sectional pre-stretching machine for longitudinally stretching each, lateral row of holes one by one (when stretching, normally the thin polymer network sheet is suitably heated and softened and then stretched in a specific proportion and cooled with cold water or other cooling medium) so as to rearrange the long bonds of polymer molecules and provide longitudinally stretched network plies with a considerably high tensile strength and form a one-way pre-stretched grid as shown schematically in FIG. 1 of the accompanying drawings; and PA1 (4) feeding the one-way pre-stretched grid into another sectional pre-stretching machine for transversely stretching each lateral row of holes one by one so as to provide transversely stretched network plies with a high tensile strength and to form the two-way pre-stretched grid as shown schematically in FIG. 2 of the accompanying drawings.
The tensile strength of a typical polymer varies with the length of the polymerized carbon chain as shown schematically in FIG. 3 of the accompanying drawings; and more carbon chain atoms of polymer, the stronger the tensile strength thereof. The polymer appears as if all are straight bonds in the drawings but in fact there is a certain angle between two adjoining carbon bonds, so such a polymer will have an extremely high stretching magnitude when subject to tension.
In producing a conventional polymer pre-stretched grid, it is intended to eliminate the angle between the carbon bonds, and through the effect of directional disposition, the tensile strength of the polymer is considerably stepped up, and the yielding strength of polymer products achievable is between 20 k-50 k psi, which is almost the same as that of mild steel, of which the yielding strength is 30 k-36 k psi.
The foregoing conventional polymer pre-stretched grid is produced by punching a thin polymer sheet molded by a molding machine and then stretching each lateral row of holes one by one to form a square or rectangular grid so as to rearrange the bonds of polymer molecules. The process of stretching (see FIG. 2A) each lateral row of holes one by one between transverse lines 512 and 515; 516 and 519; 520 and 523 or longitudinal lines 526 and 529; 530 and 533 to form the grid is rather complex and slow. Concerning the stretching of grid network plies, as shown schematically in FIG. 2A and FIG. 4 of the accompanying drawings, the crossings of longitudinal and transverse network plies between 511 and 512; 515 and 516; 519 and 520; 523 and 524; 525 and 526; 529 and 530; 533 and 534 and parts of the front and rear network plies between 512 and 513; 514 and 515; 516 and 517; 518 and 519; 520 and 521; 522 and 523; 526 and 527; 528 and 529; 530 and 531; 532 and 533 are not stretched or are not fully stretched. Therefore, there is a certain angle between adjoining carbon bonds of polymer in the position of the network plies which will still have a very high stretching magnitude when subject to tension, so the unstretched and not fully stretched parts of the front and rear network plies result in a waste of polymer material. Also, because the ,conventional polymeric pre-stretched grid is produced by punching and stretching the thin polymer sheet molded by machine, the width thereof is limited by the width of the machine (less than 6 feet normally). Therefore, when there is a need to lay the sheet on a construction site, 10 to 30 cm of the sides of adjoining two sheets have to mutually overlapped, which is wasteful not only in material but also in manpower.